1. Field of the Invention
The present invention relates to method and system for manufacturing III-V Group compound semiconductor that enable formation of a crystal growth layer of a thin film compound semiconductor of uniform thickness by vapor deposition and a III-V Group compound semiconductor.
2. Background Art
In the fabrication of various compound semiconductor devices, it is a conventional practice to use a semiconductor single crystal wafer formed by consecutively overlaying required single-crystal layers on a substrate using hydride VPE, which is a process for growing an epitaxial crystal layer using a hydrogen compound such as AsH3 or PH3 in a V Group raw material, metalorganic chemical vapor deposition (MOCVD), which is a process for growing an epitaxial crystal layer by thermally decomposing a metal organic complex, or other such process.
For example, most conventional semiconductor manufacturing systems utilizing the MOCVD process supply the feed gas into the MOCVD reactor through a single supply port. In this conventional technology, the step of conducting the feed gas from the supply port to the substrate placed in the reactor for growth of a single crystal thin-film layer thereon is performed by passing the feed gas over a relatively long distance upstream of the substrate in the reactor so as to obtain a stratified flow of the feed gas having an adequately uniform flow rate and raw material concentration.
On the other hand, there has been developed a reactor that shortens the distance between the raw material supply port and the substrate by using a planar porous plate as the supply port and placing the substrate opposite the porous plate.
In order to achieve a stratified flow of feed gas having a uniform flow rate and desired concentration with the aforesaid conventional configuration, however, it is necessary to pass the feed gas in contact with the wall of the reactor, which is at a high temperature, over a relatively long distance. This entails a number of problems, such as that the crystal purity is degraded because the heat absorbed from the wall causes premature decomposition of the raw material and that the raw material utilization efficiency is degraded.
Moreover, since the feed gas is supplied into the reactor through a single inlet appropriately formed in the reactor, the so-introduced feed gas is liable to experience unbalanced flow because of the nonuniformity of the flow path. This gives rise to various problems, including uneven thickness of the epitaxial crystal layer formed on the semiconductor substrate by the epitaxial vapor deposition. Moreover, when the planar porous plate type of supply port is adopted, regulation of the raw material supply rate within the plane is substantially impossible, making it difficult to make the thickness of the deposited film sufficiently uniform.
In the case of a semiconductor single crystal wafer used to fabricate an LED device, for example, uniform thickness of the epitaxial crystal layers is essential for minimizing wavelength and power characteristic scatter among the LEDs fabricated using the wafer. Owing to such nonuniformity of the deposited layers, however, it is difficult to form LEDs with the same characteristics over the entire semiconductor single crystal wafer. This increases cost because it makes it necessary to establish processes for, for example, checking the characteristics of the fabricated LEDs and sorting them by intended use.
When a required compound semiconductor single-crystal layer is formed by any of these different epitaxial vapor deposition processes, the quality of the epitaxial crystal layer formed strongly affects the properties of the final semiconductor device.
In order to make the thickness of an epitaxial crystal layer formed on a substrate uniform throughout, it is essential to make the flow rate of the feed gas supplied into the reactor uniform over the whole surface of the substrate. This is made difficult by the fact that the substrate mounted on the susceptor in the reactor is controlled to maintain a required growth temperature by, for example, high-frequency induction heating. As this temperature is high, at least around 600° C., the flow of the feed gas introduced into the reactor is disturbed by the heat. It is therefore extremely difficult to achieve uniform flow of the feed gas at the substrate surface. This is particularly true in a high-temperature manufacturing processes involving growth temperatures of 1,000° C. and higher, such as in the case of manufacturing a wafer for GaN-system LEDs.
In the case where individually supplied feed gases are mixed so as to conduct required mixed crystal deposition on a substrate, in order to fabricate III-V Group semiconductor single crystal wafers manufactured by consecutively overlaying required single crystal thin-film layers on a semiconductor or other appropriate substrate, the manufacture of wafers has been carried out using hydride vapor phase epitaxy (HVPE), which is a process for growing an epitaxial crystal layer using a hydrogen compound such as AsH3 or PH3 in a V Group raw material, metalorganic chemical vapor deposition (MOCVD), which is a process for growing an epitaxial crystal layer by thermally decomposing a metal organic complex, or other such process.
When a GaN-based III-V Group compound semiconductor single crystal wafer (e.g., an InGaAlN wafer) is manufactured by one of these methods, a substrate controlled to a proper temperature beforehand is set in a reactor, III Group feed gas, V Group feed gas and feed gas for doping are introduced into the reactor from an external feed gas supply source, and a mixture of these feed gases is fed onto the substrate to conduct the required crystal growth on the substrate.
However, it is known that when III-V Group compound semiconductors are manufactured by HVPE or MOCVD in the foregoing manner, the high temperature (700° C.-1,100° C.) in the reactor causes side-reactions to occur between the V Group material and the metal organic complexes constituting the III Group and/or II Group raw materials before they reach the substrate. For example, side-reactions caused by premature decomposition occur between the III Group raw material trimethyl indium (TMIn) and the V Group raw material phosphine (PH3) and between the III Group raw material trimethyl gallium (TMGa) and the V Group raw material ammonia (NH3).
When side-reactions occur between different III-V Group feed gases in this manner in the case of forming mixed crystal composed of GaN crystal and AlN crystal, the composition of the produced crystal thin film does not match the intended composition because the trimethyl aluminum (TMAl) supplied as AlN material and the bis-ethyl-cyclo-pentadienyl-magnesium ((EtCp)2Mg) supplied as dopant are consumed by side-reactions. In addition, a problem of marked decrease in the crystal deposition rate arises. When side-reactions occur, moreover, the products of the side-reactions act as nuclei that give rise to abnormal particle growth on the substrate. As this degrades crystal quality, it becomes difficult to ascertain conditions for stable and effective growth of thin-film crystal layers on the substrate, thus causing still another problem.
Further, since the side-reactions slow the rate of crystal deposition, the crystal films produced become thin. Also or instead, the products of the side-reactions deposit in large amounts at the upstream side of the reactor. This increases maintenance costs owing to the time and effort that have to be spent for the frequently required removal of the deposits.
A conventional method adopted when, for example, manufacture of high quality thin-film crystal layers is required has therefore been to suppress occurrence of side-reactions before thin-film crystal deposition by supplying the feed gases onto the substrate in the reactor individually as required.
However, when different kinds of feed gas are individually supplied onto a semiconductor or other substrate, the mixing of the supplied feed gases is insufficient. This leads to various inconveniences, such as that thin-film crystal of desired composition cannot be deposited to the prescribed thickness. In the deposition of GaN-based thin-film crystal, for example, insufficient mixing of the feed gases of course slows the deposition rate and, in addition, causes the mixed crystal to have lower content of Al, In and other III Group elements aside from Ga and, in the case of adding a II Group element as dopant, reduces the crystal deposition rate and/or decreases the efficiency of the II Group element incorporation.